This invention deals with a scanning device for emitting a laser beam and receiving a return laser beam, for example in an optical instrument called a "lidar" (acronym for "Light Detection and Ranging") which is, as is known, the equivalent of a radar in the optical wavelength range (typically between 0.1 and 15 .mu.m, or more).
In particular, lidars can be used, whether in space applications or not, for measuring atmospheric parameters or distances. In more general terms, lidars enable extended fluid masses to be characterized by compiling the results of selected parameter measurements carried out within multiple zones of these masses by a scanning device.
Such a known lidar 1 is represented in FIGS. 1 to 3. It embodies a laser source 2 with an emission axis X.times.X;
a telescope 3 with an optical axis Y--Y coplanar with the axis X--X, represented here as a block having a concave reflecting surface 4, the axis of which is coaxial with the optical axis Y--Y and at the focal point of which a detector 5 of any known appropriate type is fixed by means of a supporting structure generally shown by 5a;
a deflecting mirror 6 located at the intersection of the X--X and Y--Y axes and oriented perpendicular to their bisector so as to deflect a laser beam emitted by the laser source 2 along the X--X axis directly away from telescope 3; and
a scanning mirror 7 mounted so as to pivot under the action of a motor shown schematically by 8, around a rotation axis coaxial with the telescope axis, intercepting the Y--Y axis opposite the telescope with respect to the deflecting mirror, in such a way as to intercept the laser beam emitted by the laser source 2 and deflected by the mirror.
In practice the emission axis of the laser source 2 is arranged at 90.degree. with respect to the optical axis of the telescope and the scanning mirror is arranged at 45.degree. with respect to this axis in order to be able, during its rotational movement (continuous or oscillatory), to send the laser beam in a plane transverse to the optical axis (this is then referred to as transverse scanning) and to be able to receive a return beam in the same plane so as to deviate it towards the reflecting surface of the telescope.
It will be appreciated that only the scanning mirror can move, the other elements being fixed with respect to a supporting structure, for example a spacecraft platform (not represented) orbiting around a planet such as the Earth.
FIG. 1 shows the emission of a laser pulse whereas FIG. 2 corresponds to the reception of a return beam, deflected for example by the Earth's atmosphere.
Compensation for the delay angle caused by the rotation of the scanning mirror between the time when the laser pulse is deflected away from the instrument by the scanning mirror and the time when a return pulse enters into the instrument so as to be deflected by the scanning mirror towards the telescope, is not taken into account in the sketches shown in FIGS. 1 to 3. This compensation is achieved for example by means of an appropriate pre-inclination of the laser source emission axis relative to the orientation it should be given if the scanning mirror were to remain fixed during the interval between emission and reception. Not being in itself part of the invention, this aspect will not be described in further detail here.
It can be appreciated that in order for the laser beam (specifically, this is in practice a succession of laser pulses) to describe a conical or even (as in the previously described case of transverse scanning) plane surface, the beam emitted by the source must intercept the scanning mirror at its center. However it may be observed in FIG. 3 that when the emitted beam is reflected by the central portion of the scanning mirror, stray light problems occur, which cause this central portion to scatter parasitic rays in all directions, and in particular towards the reflecting surface of the telescope and then towards the detector 5.
This parasitic flux is difficult to asses (it depends in particular on the diffusion coefficients of the mirror and protective elements--always imperfect--intended to shield the telescope from parasitic light); nonetheless it may be sufficiently significant to saturate the telescope detector since it is normally designed in accordance with the expected return flux, several orders of magnitude lower than the emitted light flux.
The purpose of the invention is to obviate the above-mentioned drawbacks by means of an arrangement of the scanning emission-reception instrument which, without degrading the scanning quality, minimizes the parasitic light flux likely to reach the detector.
An object of this invention is therefore to provide a scanning emitter-receiver optical device which embodies an optical source adapted to emit a primary beam along an emission axis, a deflecting mirror arranged on the emission axis so as to intercept and deflect the primary beam, a telescope having an optical axis and a detector on this axis, and a scanning mirror mounted so as to rotate around a scan axis coaxial with the optical axis of the telescope and having a non-zero inclination with respect to the optical axis. The optical source is positioned opposite to the telescope with respect to the scanning mirror, the emission axis being coaxial with the telescope optical axis, the deflecting mirror being fixed to the scanning mirror and arranged at 90.degree. within a central opening provided in the scanning mirror.
According to preferred and possibly combined features of the invention the scanning mirror and the deflection mirror exhibit, with respect to the telescope optical axis, inclinations substantially equal to 45.degree.;
the optical source is a laser source;
the scanning mirror embodies a hollow tube, traversed by the emission axis, which cooperates with bearings and a drive motor;
the drive motor cooperates with the hollow tube by means of a rack and pinion assembly; and
the scanning mirror and the deflecting mirror are rotatively driven by a hollow motor.
Other objects, features and advantages of this invention will become apparent from the following detailed description of non-limiting examples thereof, taken in conjunction with the accompanying drawings.